The objective of this project is to determine the nature of lipid- protein interactions occurring at the membrane level. The study involves the use of model membranes, which permit control over the operational parameters and thus afford analysis in greater detail than is currently possible with biological membranes. Planar lipid bilayers will be used to characterize the electrical properties of the lipid- protein complex, while homogeneous vesicles and multilamellar liposomes will be employed to determine the intrinsic association constants and stoichiometry of the interaction. The protein of specific interest will be the soluble ATPase derived from S. fecalis membrane ghosts. This protein has been purified and found to interact with planar lipid bilayers to cause marked changes in the electrical properties of the membrane. Preliminary experiments with liposomes have directly demonstrated the existence of a bilayer-ATPase complex. The elucidation of the complex formation, which is possible in these relatively well defined systems, should provide insight into the nature of the lipid- protein interactions in biological membranes.